#24 Dynamic list node better API

Using `step_into_list` is the only way to get correct node and time from dynamic list (even potential `get_list_links` doesn't work because it doesn't return time) and that's very cumbersome. Perhaps the time has come to introduce a more generic `ValueInContext` type, which would contain all information necessary to compute node value (node and context). Dynamic list nodes would then simply return `std::vector<ValueInContext>`.